Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5.5	$ 4.9	$ 5.5
Interest cost	15.0	15.9	13.8
Expected return on plan assets	(19.9)	(18.9)	(19.3)
Recognized net actuarial loss	(5.6)	(6.6)	(6.3)
Amortization of prior service cost (credit)	0.2	0.2	0.2
Amount of settlement loss recognized	0.6	0.8	0.0
Net periodic benefit cost (credit)	7.0	9.5	6.5
Amounts related to discontinued operations	0.0	0.0	(14.9)
Net periodic benefit cost	7.0	9.5	(8.4)
Postretirement Benefits Other than Pensions
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.2	1.3	1.7
Interest cost	1.4	1.6	1.6
Expected return on plan assets	0.0	0.0	0.0
Recognized net actuarial loss	(0.3)	(0.6)	(0.3)
Amortization of prior service cost (credit)	(0.2)	(0.2)	(0.2)
Amount of settlement loss recognized	0.0	0.0	0.0
Net periodic benefit cost (credit)	2.7	3.3	3.4
Amounts related to discontinued operations	0.0	0.0	0.0
Net periodic benefit cost	$ 2.7	$ 3.3	$ 3.4